ACK is the "ACKnowledgement" sent by the receiving radio to the sending radio. This lets the sending radio know that the recipient is ready for the next block of data. Distance is determined by how long it takes a packet to move from one radio to another.
In Point-to-Point (PTP) mode, airMAX AC devices will automatically switch from hardware ACK to software ACK (NOACK), with no configuration required. In Point-to-Multipoint (PtMP) mode however, NOACK isn't supported as such, and there will be distance limitations.
PtMP Mode Distance Limitations
|20/40/80 MHz||22.5 km (14 mi)|
|30/60 MHz||15 km (9.3 mi)|
|50 MHz||18 km (11.2 mi)|
|10 MHz||37.5 km (23.3 mi)|